[CMake] Basic example of adding an MSVC project with ExternalProject_Add?

[Clifford Yapp]

I would like to use the ExternalProject_Add command to add a
pre-existing MSVC build system into a CMake generated MSVC project,
but I can't seem to find any examples of this and I haven't the least
idea what commands to supply for CONFIGURE/BUILD/etc. in order to
bring in the MSVC files.  Can anyone point me to a working example or
documentation spelling out how to do this?  Bonus points if it has
older project files that get updated to a newer Visual Studio.

Cheers,
CY